(207d) Carboxylation of Propylene Oxide to Propylene Carbonate

Polycarbonates, a class thermoplastic polymer, have wide ranging applications in automotive & transportation, construction, packaging, consumer goods, optical media, electrical & electronics and medical devices. Conventionally, manufacturing of polycarbonates involves phosgenation of bis-phenol-A, which has drawbacks due to toxic phosgene and corrosive products like HCl in stoichiometric amounts. Alternative non-phosgene route for polycarbonates involves carboxylation of propylene oxide as a key step. This is also an example of CO₂ utilization for value added chemicals. Cyclic carbonates are also widely used as aprotic solvents, anti-foaming agents, antifreeze, plasticizers, and monomer for various commodity polymers. Several studies reported carboxylation of epoxides employing either homogeneous or heterogeneous catalysts. Reported heterogeneous catalysts include metal oxides, zeolites, polymer supported quaternary onium salts, ion exchange resins, and polymer supported ionic liquids. However, the main challenges in this reaction are low catalyst activity, and high CO₂ pressure. In this study, we systematically investigated the influence of solvents on ion exchange resin catalyzed carboxylation of propylene oxide with the aim of enhancing the activity and selectivity. Intrinsic kinetics of carboxylation of propylene oxide has also been studied in a batch slurry reactor to understand the effect of reaction conditions and develop a suitable rate equation.

The experiments for solvent effects and kinetic studies were carried out in a stirred pressure reactor with 100 cm³ capacity with provisions for control of agitation speed, temperature and sampling of liquids. In these experiments, the CO₂ pressure in the reactor was kept constant by continuous supply through a CO₂ reservoir using a constant pressure regulator such that the temporal reaction progress was followed from the pressure decrease in the reservoir. At the end of each experiment, liquid products were analyzed for PO and PC to assess the material balance. The effect of solvents and the order of activity is: Methanol > Ethanol > Isopropyl alcohol > Dimethyl formamide > N-methyl-2-pyrrolidone > Propylene carbonate > Dimethyl carbonate and it was found that using methanol as a solvent rate of carboxylation were enhanced by a factor of 3. The polar protic solvents (such as linear alcohols) are strong nucleophiles, strong hydrogen-bond donors and acceptors and interact strongly with the electron-deficient electrophiles, epoxides and CO₂. In contrast, polar aprotic solvents are weak nucleophiles compared to the polar protic solvents resulting in low activity. These conclusions, based on CO₂ solubility measurements in methanol and PC solvents along with kinetic studies of PO carboxylation, will be presented and discussed.